This invention generally relates to a sootblower device for cleaning interior surfaces of large-scale combustion devices. More specifically, this invention relates to new designs of nozzles for a sootblower lance tube providing enhanced cleaning performance.
Sootblowers are used to project a stream of a blowing medium, such as steam, air, or water against heat exchanger surfaces of large-scale combustion devices, such as utility boilers and process recovery boilers. In operation, combustion products cause slag and ash encrustation to build on heat transfer surfaces, degrading thermal performance of the system. Sootblowers are periodically operated to clean the surfaces to restore desired operational characteristics. Generally, sootblowers include a lance tube that is connected to a pressurized source of blowing medium. The sootblowers also include at least one nozzle from which the blowing medium is discharged in a stream or jet. In a retracting sootblower, the lance tube is periodically advanced into and retracted from the interior of the boiler as the blowing medium is discharged from the nozzles. In a stationary sootblower, the lance tube is fixed in position within the boiler but may be periodically rotated while the blowing medium is discharged from the nozzles. In either type, the impact of the discharged blowing medium with the deposits accumulated on the heat exchange surfaces dislodges the deposits. U.S. Patents which generally disclose sootblowers include the following, which are hereby incorporated by reference U.S. Pat. Nos. 3,439,376; 3,585,673; 3,782,336; and 4,422,882.
A typical sootblower lance tube comprises at least two nozzles that are typically diametrically oriented to discharge streams in directions 180xc2x0 from one another. These nozzles may be directly opposing, i.e. at the same longitudinal position along the lance tube or are longitudinally separated from each other. In the latter case, the nozzle closer to the distal end of the lance tube is typically referred to as the downstream nozzle. The nozzle longitudinally furthest from the distal end is commonly referred to as the upstream nozzle. The nozzles are generally but not always oriented with their central passage perpendicular to and intersecting the longitudinal axis of the lance tube and are positioned near the distal end of the lance tube.
Various cleaning mediums are used in sootblowers. Steam and air are used in many applications. Cleaning of slag and ash encrustations within the internal surfaces of a combustion device occurs through a combination of mechanical and thermal shock caused by the impact of the cleaning medium. In order to maximize this effect, lance tubes and nozzles are designed to produce a coherent stream of cleaning medium having a high peak impact pressure on the surface being cleaned. Nozzle performance is generally quantified by measuring dynamic pressure impacting a surface located at the intersection of the centerline of the nozzle at a given distance from the nozzle. In order to maximize the cleaning effect, it is desired to have the stream of compressible blowing medium fully expanded as it exits the nozzle. Full expansion refers to a condition in which the static pressure of the stream exiting the nozzle approaches that of the ambient pressure within the boiler. The degree of expansion that a jet undergoes as it passes through the nozzle is dependent, in part, on the throat diameter (D) and the length of the expansion zone within the nozzle (L), commonly expressed as an L/D ratio. Within limits, a higher L/D ratio generally provides better performance of the nozzle.
Classical supersonic nozzle design theory for compressible fluids such as air or steam require that the nozzle have a minimum flow cross-sectional area often referred to as the throat, followed by an expanding cross-sectional area (expansion zone) which allows the pressure of the fluid to be reduced as it passes through the nozzle and accelerates the flow to velocities higher than the speed of sound. Various nozzle designs have been developed that optimize the L/D ratio to substantially expand the stream or jet, as it exits the nozzle. Constraining the practical lengths that sootblower nozzles can have is a requirement that the lance assembly must pass through a small opening in the exterior wall of the boiler, called a wall box. For long retracting sootblowers, the lance tubes typically have a diameter on the order of three to five inches. Nozzles for such lance tubes cannot extend a significant distance beyond the exterior cylindrical surface of the lance tube. In applications in which two nozzles are diametrically opposed, severe limitations in extending the length of the nozzles are imposed to avoid direct physical interference between the nozzles or an unacceptable restriction of fluid flow into the nozzle inlets occurs. In an effort to permit longer sootblower nozzles, nozzles of sootblower lance tubes are frequently longitudinally displaced. Although this configuration generally enhances performance by facilitating the use of nozzles having a more ideal L/D ratio, it has been found that the upstream nozzle exhibits significantly better performance than the downstream nozzle. Thus, an undesirable difference in cleaning effect results between the nozzles.
Initially, low performance of the downstream nozzle was attributed to the loss of static pressure associated with the fluid flow passing around the bluff body presented by the upstream nozzle in the form of the cylindrical projection of the nozzle into the lance tube interior. However, experiments conducted revealed that even when the upstream nozzle is moved radially outward to present no obstruction to the flow through the lance tube, the performance of the downstream nozzle did not significantly improve. The low performance of the downstream nozzle is believed to be due, in a significant manner, to the stagnation area created in the distal end of the conventional lance tube. A typical lance tube end or xe2x80x9cnozzle blockxe2x80x9d has a rounded, hemispherical distal end surface. Since the downstream nozzle penetrates the nozzle block before the distal end hemispherical end surface, an internal volume exists beyond the downstream nozzle. Accordingly, a significant portion of the cleaning fluid approaching the downstream nozzle is forced to flow past the nozzle inlet and come to a stagnation condition at the distal end of the lance tube, and then re-accelerate to enter the nozzle. Furthermore, the back streams returning from the distal end are colliding with the forward streams at the downstream nozzle inlet leading to greater hydraulic losses and most importantly distorting the flow distribution into the nozzle. The hydraulic losses associated with the stagnation conditions at the distal end and at the nozzle inlet coupled with the flow mal-distribution which, based on concepts developed in connection with this invention, were believed, in large part, responsible for the low performance of the downstream nozzle. Therefore, there is a need in the art to provide a new lance tube design that will substantially increase the performance of the downstream nozzle.
In accordance with this invention, improvements in nozzle design are provided which provide enhanced performance of the downstream nozzle. In each case according to this invention, the nozzle block is formed to substantially eliminate the stagnation within the lance tube area beyond the downstream nozzle found in the prior art designs. Another beneficial feature of this invention involves streamlining at the upstream nozzle which minimizes the disruption to flow of cleaning medium to the downstream nozzle.
Briefly, a first embodiment of the present invention includes a downstream nozzle at the distal end of the lance tube with a converging channel formed in the interior of the lance tube to direct the flow of the cleaning medium passing the upstream nozzle and directing the flow to the downstream nozzle. The converging channel substantially eliminates the stagnation volume of the distal end of the conventional lance tube. This has the benefit of reducing hydraulic losses and improving the degree of uniformity of flow velocity at the throat, which in turn enhances the flow expansion and the conversion of static energy into kinetic energy.
The second embodiment of the present invention has an interior surface substantially identical to the first embodiment. However, the second embodiment nozzle block has a thin wall configuration which reduces the mass of the nozzle block.
A third embodiment of the present invention includes an airfoil body around the outside surface of the upstream nozzle. By providing streamline design of the outer surface of the upstream nozzle, the flow disturbances associated with the upstream nozzle is minimized.
A fourth embodiment of the invention features an upstream nozzle with its inlet end tipped toward the flow of the cleaning medium flowing through the lance tube.
In a fifth embodiment, the upstream nozzle features a longitudinal axis perpendicular to the longitudinal axis of the lance tube with the nozzle inlet tipped toward the flow of the blowing medium.
In a sixth embodiment in accordance with the teaching of the present invention provides for the design of the upstream nozzle having its outlet end flush with the body of the lance tube.